1. Field of the Invention
The present invention relates to a negative feedback amplifier used in a high-speed optical fiber communication system and the like, and especially the present invention relates to technology for increasing the reliability of the negative feedback amplifier.
2. Description of the Related Art
Electrostatic Discharge (ESD) causing damage to a semiconductor device occurs in a circuit or an electrical insulator, which is different from the semiconductor device, and which is for example installed in equipment having the semiconductor device, or which may be a body of a human who handles the semiconductor device, and the like. The ESD is an important factor which affects the reliability of the semiconductor device. Accordingly, it is preferable to sufficiently increase threshold voltage to the ESD, for the purpose of securing stable operation.
A transistor for amplification positioned in an input section especially sustains the damage from ESD. The damage is generally considered as heat damage. In other words, electric current flows through a junction between an electrode metal plate and the semiconductor device due to the addition of ESD. Since the electric current increases the temperature of the junction, the resistance thereof decreases and causes overheating. Then, the junction is damaged by melting.
In order to protect the semiconductor device from the ESD, a method by which a Zener diode or the like is disposed in the input or output of an amplifier is conventionally adopted, where the Zener diode is designed so as to operate with a voltage more than a predetermined voltage
FIGS. 1A to 1C are explanatory views of a conventional electrostatic protective circuit for the amplifier.
In FIG. 1A, diodes 4 and 5 are connected between an input terminal of the amplifier (AMP) 1 to be protected and a positive power terminal 2, and between the input terminal and a negative power terminal 3, respectively, in such a manner that the diodes 4 and 5 are biased in opposite directions. Accordingly, when the ESD is applied to the input terminal, the amplifier 1 is protected from the ESD because the diode 4 or 5 is conducting. However, there is a problem that the capacitance of the diodes 4 and 5, connected between the input terminal of the amplifiers 1 and the power terminals 2 and 3, adversely affects high-frequency characteristics.
Japanese Patent Laid-Open Publication No. 2001-110993 discloses electrostatic protective circuits which protect the semiconductor device from the ESD without impairing the high-frequency characteristic. FIGS. 1B and 1C show these electrostatic protective circuits.
In the electrostatic protective circuit shown in FIG. 1B, cathodes of protective diodes 6 and 7 are connected to the input terminal of the amplifier 1, which is to be protected and is composed of High Electron Mobility Transistors (HEMTs). The anode of the diode 6 is connected to the positive power terminal 2 while the anode of the diode 7 is connected to the negative power terminal 3. These diodes 6 and 7 are composed of a plurality of Schottky diodes connected in series, in such a manner as to be in an off-state when normal input signals are inputted. The Schottky diode includes a HEMT formed in the same process as the amplifier 1. In the Schottky diode, the source and the drain of the HEMT short out.
In the electrostatic protective circuit shown in FIG. 1C, the cathodes of the diodes 6 and 7 are not directly connected to the input terminal, but connected via a diode 8. The configuration of the electrostatic protective circuits like this makes it possible to improve the ESD resistance without impairing the characteristics of a high-frequency device.
The conventional electrostatic protective circuit, however, has the following problem.
In a high-speed optical fiber communication system, for example, optical signals carried by optical fibers are converted into current signals by a photoelectric conversion element such as a photodiode and the like. A negative feedback amplifier converts the current signals into voltages, and amplifies them. The negative feedback amplifier requires characteristics such as low noise, a high dynamic range, high bandwidth, and high gain. An amplifier called a trans impedance type amplifier, in which a feedback resistor is connected between an input and an output, is generally used as the negative feedback amplifier.
By the way, the band width fw of the negative feedback amplifier is expressed by the following equation:fw=A/(2π·Rf·Cin),wherein A is the open-loop gain of the amplifier, Rf is feedback resistance, and Cin is input capacitance. The input capacitance Cin is the total of “the junction capacitance of the photodiode and the like”, “the input capacitance of the amplifier”, and “stray capacitance added by mounting”.
Taking a case of a negative feedback amplifier operating at the level of 10 Gbps, for example, the junction capacitance of a generally used photodiode is about 150 fF (femto-Farad), the input capacitance of an amplifier is 50 to 100 fF, and the stray capacitance is several tens of fF. The total input resistance Cin of these is approximately 200 to 300 fF.
When the electrostatic protective circuits shown in FIGS. 1A to 1C are added in the negative feedback amplifier, the input resistance Cin is more than doubled because the capacitance of the protective diode itself is more than several hundreds of fF. Thus, it is difficult to obtain the desired band width fw.